Rental vehicles from rental agencies typically do not have any valve caps on the valve stems of the vehicle wheels of the rental vehicles. The absence of valve caps is not because the cars were built this way by the different original equipment manufacturers (OEMs), but because rental agency employees do not want to handle the valve caps. Every time a vehicle is returned to the rental office, the agency is obligated, as a normal check up of the vehicle, to verify the pressure in each tire of the returned vehicle even if this vehicle could be equipped with an individual pressure display of each tire on the instrument panel of the vehicle.
In these situations, a technician has to remove the valve caps, one at a time, use a tire gauge and make a manual note with the corresponding vehicle identification number (VIN) that the vehicle has the correct pressure in the tires. Knowing that the technician will have to remove the caps again when the vehicle comes back from the next customer, the technician never bothers to put the caps on in order to save time, or over-inflates the tire to minimize having to deal with future pressure checks and re-inflations.
The valve cap of a valve stem has different critical functions. First, it protects the valve core mechanism of the valve stem against dirt, salt, and debris from the road and other sources that could eventually damage the valve core mechanism. If the valve core mechanism is damaged, its spring and seal functions could lead to potential leaks. If debris goes though the open valve mechanism during tire inflation, this debris could interfere with the valve core sealing cup mechanism leading to leaks.
Many late model vehicle valve caps, especially those equipped with tire pressure monitoring systems (TPMS), also have an O-ring seal that seals the valve stem from debris, water and so forth. This further improves the tightness of the valve stem assembly against air leaks by acting as a secondary seal. If the valve cap is not placed on the valve stem, or if some debris are located around the valve core mechanism spring, or if some debris are stuck on the cup seal, some potential air leaks could happen leading eventually to a leaking or flat tire. Furthermore, failure to reinstall valve caps exposes the end of the valve stem and valve core to potentially highly corrosive brine solutions used for ice melting on highways, automotive/wheel cleaning chemicals, and so forth.
The non-replacement of the caps potentially results in the above-mentioned problems. Consequently, some user dissatisfaction has resulted from these previous instances where the cap is not reinstalled.
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